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A common practice in laboratory or outdoor weathering tests, is to time the exposure in UV Joules instead of hours or days. The assumption is that Joules of radiant UV exposure give a relatively reliable index of the degradation forces impinging on the test specimens. However, timing in Joules can be extremely misleading. Joules do not reflect variations in degradation caused by differences in exposure to moisture, temperature, or wavelength spectrum of the light source. Characterization and control of these other parameters is oftenmore important than Joules of radiant dosage. Controlled tests were conducted, varying either temperature, moisture, or wavelength spectrum while holding other conditions constant. In these tests, replicate specimens exposed to identical radiant dosage in Joules showed variations of over 500% in gloss loss and yellowing. This effect was observed in several different polymers. The conclusion is that for UV exposure tests, measuring radiant dosage may be a worthwhile control, but it is by no means sufficient as a description of the test conditions.
Ultraviolet, radiant dosage, Joules, weathering, durability, photodegradation
President, Q-Panel Company, Westlake, OH